1. Field of the Invention
The present invention relates generally to a mobile communication system supporting a multimedia service including voice and data services, and in particular, to an apparatus and method for controlling power of a forward common power control channel.
2. Description of the Related Art
An IS-2000 CDMA (Code Division Multiple Access) mobile communication system, a typical mobile communication system, supports only a voice service. However, with the development of the communication technology and at the request of users, research has been carried out on a mobile communication system for supporting a data service. For example, an HDR (High Data Rate) communication system has been proposed to support only a high-speed data service.
The existing mobile communication systems are classified into one mobile communication system for supporting only the voice service and another mobile communication system for supporting only the data service. That is, although the mobile communication system is required to support both the voice service and the data service, the conventional mobile communication system is designed to support only the voice service or the data service. Therefore, there have been long demands for a mobile communication system capable of supporting both the voice service and the data service. To meet the demands, a 1×EV-DV (Evolution Data and Voice) mobile communication system has been proposed recently.
FIG. 1 illustrates forward and reverse power control-related channels in a conventional 1×EV-DV mobile communication system. Referring to FIG. 1, sectors (or base stations) #1 and #2 each measure a reverse pilot channel (R-PICH) from a mobile (or Mobile Station (MS)) to estimate a signal-to-noise ratio Ep/Nt, and compare the signal-to-noise ratio with a reference value for outer loop power control (or outer loop set point). If the estimated signal-to-noise ratio is larger than or equal to the reference value, the base stations send power-down commands to the mobile over a forward common power control channel (F-CPCCH). Otherwise, if the estimated signal-to-noise ratio is lower than the reference value, the base stations send power-up commands to the mobile over the forward common power control channel.
A soft handoff performed by the mobile is illustrated in FIG. 1. During a soft handoff, the mobile receives forward common power control channels CPCCH1 and CPCCH2 from two or more sectors #1 and #2. If at least one of the received CPCCHs includes a power-down command, the mobile decreases its transmission power. However, the mobile increases its transmission power, only when power-up commands are provided from all sectors.
In FIG. 1, transmission power of the F-CPCCH is controlled through a channel quality indicator channel (CQICH) transmitted over a reverse link. The CQICH transmits a carrier-to-interference ratio (C/I) value of a common pilot channel, measured by the mobile for a specific sector, to the sector. In general, the CQICH is used by the mobile to measure forward common pilot channels transmitted from a plurality of sectors neighboring to the mobile, and transmit a C/I value (or a forward channel condition information) of a best sector having the highest C/I value among the measured common pilot channels to the corresponding sector. Upon receiving the CQICH, the sector determines power to be allocated to the CPCCH based on the C/I value of the forward common pilot channel measured by the mobile.
When forward and reverse power control is performed during the soft handoff illustrated in FIG. 1, certain problems may occur. That is, a forward power control problem for the two or more CPCCHs may occur. The mobile in a soft handoff state is provided with CPCCHs transmitted from two or more sectors.
To better illustrate this problem, during the soft handoff, although the CPCCHs are transmitted to the mobile from two or more sectors, the mobile measures a common pilot channel from only one sector, and transmits the measurement result over the CQICH. If it is assumed in FIG. 1 that the mobile performs a soft handoff between two sectors #1 and #2, and that the sector #1 has a better forward channel environment than the sector #2, then the CQICH of the mobile transmits a measured value of only the common pilot channel from the sector #1 to the sector #1, a best sector. Then, the sector #1, the best sector, receives the CQICH from the mobile, and determines transmission power of CPCCH to be transmitted based on a C/I value of a forward channel included in the received CQICH. However, since the sector #2 has a worse forward channel environment than the sector #1, the CQICH has no C/I value of the forward channel to be used when determining transmission power of the CPCCH.
In addition, if an erasure process occurs because reception power of the CQICH is lower than a predetermined threshold, regardless of the soft handoff, the base station cannot determine transmission power of CPCCH symbols to be allocated to the mobile.